Since the 3rd century for more than a thousand years chemistry has been thought of as a complicated, hard-to-predict science. Efforts to improve even a part of its unpredictable character are said to have born fruit first of all in the success of the \" electronic theory \". This was founded mainly by organic chemists , such as Fry, Stieglitz, Lucas, Lapworth and Sidgwick, brought to a completed form by Robinson and Ingold, and developed later by many other chemists. 1 In the electronic theory, the mode of migration of electrons in molecules is noted and is considered under various judgements. For that purpose, a criterion is necessary with respect to the number of electrons which should originally exist in an atom or a bond in a molecule. Therefore, it can be said to be the concept by Lewis of the sharing of electrons that has given a firm basis to the electronic theory. 2 In the organic electronic theory, the chemical concepts such as acid and base, oxidation and reduction and so on, have been conveniently utilized from a long time ago. Furthermore, there are terms centring closer around the electron concept, such as electrophilicity and nucleophilicity, and electron donor and acceptor both being pairs of relative concepts. One may be aware that these concepts can be connected qualitatively to the scale of electron density or electric charge. In the electronic theory, the static and dynamic behaviours of molecules are explained by the electronic effects which are based on nothing but the distribution of electrons in a molecule. The mode of charge distribution in a molecule can be sketched to some extent by the use of the electronegativity concept of atoms through organic chemical experience. At the same time, it is given foundation, made quantitative , and supported by physical measurements of electron distribution and theoretical calculations based on quantum theory. The distribution of electrons or electric charge-with either use the result is unchanged-in a molecule is usually represented by the total numbers (generally not integer) of electrons in each atom and each bond, and it was a concept easily acceptable even to empirical chemists as having a tolerably realistic meaning. Therefore, chemists employed the electron density as a fundamental concept to explain or to comprehend various phenomena. In particular, for the purpose of promoting chemical investigations, researchers usually rely upon the analogy through experience, and the electron density …

https://science.sciencemag.org/content/sci/218/4574/747.full.pdf

Role of Frontier Orbitals in Chemical Reactions

Recently, there has been growing interest in research into the role of plant-derived antioxidants in food and human health. The beneficial influence of many foodstuffs and beverages including fruits, vegetables, tea, coffee, and cacao on human health has been recently recognized to originate from their antioxidant activity. For this purpose, the most commonly methods used in vitro determination of antioxidant capacity of food constituents are reviewed and presented. Also, the general chemistry underlying the assays in the present paper was clarified. Hence, this overview provides a basis and rationale for developing standardized antioxidant capacity methods for the food, nutraceutical, and dietary supplement industries. In addition, the most important advantages and shortcomings of each method were detected and highlighted. The chemical principles of these methods are outlined and critically discussed. The chemical principles of methods of 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulphonate) radical (ABTS·+) scavenging, 1,1-diphenyl-2-picrylhydrazyl (DPPH·) radical scavenging, Fe3+–Fe2+ transformation assay, ferric reducing antioxidant power (FRAP) assay, cupric ions (Cu2+) reducing power assay (Cuprac), Folin-Ciocalteu reducing capacity (FCR assay), peroxyl radical scavenging, superoxide anion radical (O
 2
 ·−
 ) scavenging, hydrogen peroxide (H2O2) scavenging, hydroxyl radical (OH·) scavenging, singlet oxygen (1O2) quenching assay and nitric oxide radical (NO·) scavenging assay are outlined and critically discussed. Also, the general antioxidant aspects of main food components were discussed by a number of methods which are currently used for detection of antioxidant properties food components. This review consists of two main sections. The first section is devoted to main components in the foodstuffs and beverages. The second general section is some definitions of the main antioxidant methods commonly used for determination of antioxidant activity of components in the foodstuffs and beverages. In addition, there are given some chemical and kinetic basis and technical details of the used methods.

Antioxidant activity of food constituents: an overview

Tetrahedron report number 163

Publisher Summary This chapter discusses the scope of Ziegler catalysis; stereoselectivity, kinetics, and mechanism of Ziegler catalysis; homogeneous Ziegler–Natta catalysts; and side reactions in homogeneous catalysts. Ziegler catalysis involves rapid polymerization of ethylene and α-ole-fins with the aid of catalysts based on transition-element compounds, normally formed by the reaction of a transition-element halide or alkoxide or alkyl or aryl derivative with a main-group element alkyl or alkyl halide. There are thousands of patents involving every combination of pure or mixed main-group alkyls with transition-element compounds, each claiming advantages. The result of the early work led to the development of “second-generation” Ziegler catalysts. Polymers produced with unmodified Ziegler catalysts showed extremely high molecular weight and broad distribution, and in some cases, there was evidence for “living polymer.” All homogeneous catalyst systems for ethylene polymerization become heterogeneous when polyethylene is formed. On using vanadium-based homogeneous catalysts, polymers consisting of syndiotactic stereo blocks and stereo-irregular blocks are obtained. Very high stereoselectivity is observed for racemic 4-methyl- 1-hexene and racemic 3,7-dimethyl-1-octene, where the asymmetric carbon atom is in the α-position relative to the double bond. Stereoselectivity is caused by the chirality of the catalytically active center, and not by chiral atoms in the growing chain. It must be concluded from the results that reactions take place, which change the number of active sites present, due to the different behavior of the polymers in solution. Study of these new catalysts is intensive. After a short induction period, the activity of polymerization increases as a function of the monomer concentration.

Ziegler-Natta Catalysis

Supported vanadium oxide catalysts, containing surface vanadia species on oxide supports, are extensively employed as catalysts for many hydrocarbon oxidation reactions. This paper discusses the current fundamental information available about the structure and reactivity of surface vanadia species on oxide supports: monolayer surface coverage, stability of the surface vanadia monolayer, oxidation state of the surface vanadia species, molecular structures of the surface vanadia species (as a function of environment and catalyst composition), acidity of the surface vanadia species and reactivity of the surface vanadia species. Comparison of the molecular structure and reactivity information provides new fundamental insights into the catalytic properties of surface vanadia species during hydrocarbon oxidation reactions: (1) the role of terminal VO, bridging VOV and bridging VO-support bonds, (2) the number of surface vanadia sites required, (3) the influence of metal oxide additives, (4) the influence of surface acidic and basic sites, (5) the influence of preparation methods and (6) the influence of the specific oxide support phase. The unique physical and chemical characteristics of supported vanadia catalysts, compared to other supported metal oxide catalysts, for hydrocarbon oxidation reactions are also discussed.

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Structure and reactivity of surface vanadium oxide species on oxide supports

The role of the V=O bond in V2O5 in the adsorption of several gases was studied by measuring the electron spin resonance (ESR) and the infrared (IR) spectra. The ESR spectra of V2O5 supported on γ-Al2O3 were similar to those of VOSO4 on γ-Al2O3. When such electron-donating gases as CO, SO2 and C2H4 were adsorbed on this V2O5-γ-Al2O3, the signal intensity of ESR increased and the splitting of the h.f.s. became clear. Upon treatment with an electron-accepting gas such as O2, a reverse change was observed. In the case of V2O5 supported on SiO2, similar ESR spectra were observed by the adsorption of CO after a mild reduction. The IR spectra of V2O5 have a sharp absorption peak at 1023 cm−1 due to the stretching vibration of the (V=O)3+ bond; this peak became broader by adsorption of such gases as CO and SO2. Simultaneously, the center of the peak shifted to the red region and a weak hump appeared at 980–990 cm−1, probably due to (V=O)2+. These results of the ESR and IR measurements reveal that these electron-...

Spectroscopic Studies of Catalysis by Vanadium Pentoxide

The behavior of oxygen, O2− and O− radicals adsorbed on the partially reduced V2O5–SiO2 catalysts has been investigated. Oxygen is adsorbed mainly in a molecular state at room temperature, the sum of amounts of paramagnetic adsorbed species O2− and O− being less than 10% of total amount of adsorbed oxygen. High degree of reduction, high content of V2O5 and low pressure of oxygen at adsorption equilibrium are favorable for the formation of O− radical. It was confirmed by the quantitative analysis of reaction products and the adsorbed species that O2−, not O−, is the active species in the oxidation of propylene at room temperature. A mechanism involving an intermediate of an epoxide structure was proposed.

On the Behavior of O2− and O− Radicals over V2O5–SiO2 Catalyst

The interactions between vanadium(V) oxide and carriers were investigated in the vanadium oxide catalysts supported on silica and γ-alumina by measuring X-ray diffraction patterns, ESR spectra, solubilities into 0.3 N ammoniacal solutions and reduction rates by benzene. A part of vanadium oxide on the carriers was insoluble in an ammoniacal solution, while unsupported vanadium oxide was easily soluble, and the reduction rates of the catalysts were remarkably affected by the carriers and the vanadium oxide contents. ESR spectra revealed that the vanadium oxide was stabilized by interaction with carriers as a square pyramidal one (VO5) and a tetrahedral one (VO4) on the surfaces of γ-alumina and silica, respectively. Massive vanadium(V) oxide crystals grew on these interacted layers with the increase of the oxide contents.

Some Physico-chemical Properties of Vanadium Oxide Supported on Silica or γ-Alumina

Die Titelspezies (II), in wasrig alkalischer Losung in einer H,-Atmosphare gebildet, katalysiert die Cyanierung von Vinylhalogeniden wie (I) bzw. (IV).

Kimio Tarama

Papers

Spectroscopic Studies of Catalysis by Vanadium Pentoxide

On the Behavior of O2− and O− Radicals over V2O5–SiO2 Catalyst

Some Physico-chemical Properties of Vanadium Oxide Supported on Silica or γ-Alumina

Stereoselective cyanation of vinyl halides catalyzed by tetracyanocobaltate(I)

Theoretical studies of hydrogen molecule adsorption on flat and stepped platinum surfaces